The present invention relates to a transmission, and more particularly relates to a transmission which incorporates a gear transmission mechanism within a main transaxle casing and an overdrive mechanism within an overdrive casing fixed to said main transaxle casing.
A typical form of automatic transmission system that is produced nowadays for a front engine front wheel drive automotive vehicle or a so called FF configuration automotive type vehicle is configured as a main transaxle casing with an overdrive casing secured to the side of the transaxle casing by bolts or the like. Within the transaxle casing there is provided a gear transmission mechanism, which includes one or more hydraulic fluid pressure actuated friction engaging mechanisms, and which is selectable between a plurality of speed stages (typically three or four forward speed stages and one reverse speed stage) according to selective supply of hydraulic fluid pressure to said hydraulic fluid pressure actuated friction engaging mechanisms. Further, within the overdrive casing there is provided an overdrive mechanism, which again includes one or more hydraulic fluid pressure actuated friction engaging mechanisms, and which is again selectable between a plurality of speed stages (typically in fact two speed stages, one of which is a directly connected speed stage and the other of which is an overdrive or speed increasing speed stage) according to selective supply of hydraulic fluid pressure to said hydraulic fluid pressure actuated friction engaging mechanisms.
The advantage of this form of constructional configuration is that during mass production of an automotive vehicle utilizing such an automatic transmission system it is possible to produce two models of vehicle, one of these models incorporating both the gear transmission mechanism incorporated in the transaxle casing and also the overdrive mechanism incorporated in the overdrive casing as fixed to the transaxle casing, and the other of these models incorporating only the gear transmission mechanism incorporated in the transaxle casing; the former of these models being thus equipped with an automatic transmission system and with an overdrive capability and being the more deluxe of the two models, and the latter of these models being only equipped with the standard automatic transmission system without an overdrive capability and being the less deluxe of the two models. In fact, it is a typical feature of such a transmission system that the differential or final drive mechanism is also housed in the transaxle casing which houses the gear transmission mechanism. Thus, in the case of the first of the models specified above which incorporates an overdrive mechanism, the overdrive mechanism which is housed in the overdrive casing secured to the transaxle casing by bolts or the like receives supply of rotational power from the output side of the gear transmission mechanism and provides supply of rotational power to the input side of the differential mechanism; while on the other hand, in the case of the second of the models specified above which incorporates no overdrive mechanism, no overdrive casing is provided as secured to the transaxle casing, but instead some form of blanking off plate is similarly secured by bolts or the like to said transaxle casing in the place where the overdrive casing was fixed in the case of the first model described above, and the output side of the gear transmission mechanism is directly coupled to the input side of the differential mechanism by some form of direct connection means, so that said output side of the gear transmission mechanism directly provides supply of rotational power to said input side of the differential mechanism. This constructional scheme thus allows for the same transaxle assembly to be utilized in both the two models described above--the model including an overdrive unit and the model not including an overdrive unit--and this provides for considerable economy in manufacture.
As stated above, it is usual for the gear transmission mechanism and the overdrive mechanism both to be hydraulically operated, and each to include one or more hydraulic fluid pressure actuated friction engaging mechanisms, selective supply of hydraulic fluid pressure to which selects the desired speed stage. Thus, of course, a hydraulic fluid pressure control system for the gear transmission mechanism is provided, and also a hydraulic fluid pressure control system for the overdrive mechanism is provided. Now, as is per se well known in the art of such hydraulic fluid pressure control systems, both of these control systems typically comprise one or more hydraulic fluid accumulators, which are typically used for cushioning and absorbing sudden surges of hydraulic fluid pressures which are selectively supplied as actuating hydraulic fluid pressures to various ones of the hydraulic fluid pressure operated friction engaging mechanisms (although alternative possibilities exist).
Now, in the prior art, all these hydraulic fluid pressure accumulators have been housed within the transaxle casing; i.e., the hydraulic fluid pressure accumulator or accumulators which are used for cushioning and absorbing sudden surges of hydraulic fluid pressures which are selectively supplied as actuating hydraulic fluid pressures to various ones of the hydraulic fluid pressure operated friction engaging mechanisms which are incorporated in the gear transmission mechanism are housed within the transaxle casing; and also the hydraulic fluid pressure accumulator or accumulators which are used for cushioning and absorbing sudden surges of hydraulic fluid pressures which are selectively supplied as actuating hydraulic fluid pressures to various ones of the hydraulic fluid pressure operated friction engaging mechanisms which are incorporated in the overdrive mechanism are housed within the transaxle casing.
Now, each of these hydraulic fluid pressure accumulators is relatively large in volume, and accordingly occupies quite a large space within the transaxle casing. Because all these hydraulic fluid pressure accumulators are housed in the transaxle casing, in the prior art, therefore, it becomes difficult to make the transaxle casing compact. Particularly in the case of a modern type front transverse engine front wheel drive automotive vehicle, compactness of the transmission as a whole, and particularly reduction of the axial length of the transmission, is a very important design objective. The disadvantage that has arisen, with a construction as specified above that has been used in the prior art wherein all these hydraulic fluid pressure accumulators are housed in the transaxle casing, is that, when as described above a constructional scheme is implemented which allows for the same transaxle assembly to be utilized in both of two models, one model including an overdrive unit and one model not including an overdrive unit, then in the case that the transaxle assembly is being utilized not in conjunction with an overdrive unit the space in the transaxle casing which is devoted to the hydraulic fluid accumulator or accumulators associated with the overdrive unit is effectively waste space. In view of the importance of maximizing compactness and minimizing axial length of the transmission system as a whole in all of its configurations, this is an important disadvantage.
Another disadvantage that has occurred in connection with the prior art constructional scheme in which all of the hydraulic fluid pressure accumulators, including the accumulator or accumulators pertaining to the overdrive unit, are housed in the transaxle casing, is that when the transaxle casing is utilized in its configuration in conjunction with an overdrive unit then the hydraulic fluid pathway which leads to each of the accumulators pertaining to the overdrive unit from the overdrive unit tends to be rather long; and this can give rise to difficulties connected with the proper control operation of the overdrive unit.